This invention relates to electromagnetic sensing apparatus and in particular to a method and apparatus for the non-contacting measurement of earth material electrical and magnetic properties with respect to depth below surface and position.
Portable devices currently available for in-situ direct estimation of nearby earth materials"" apparent conductivity will be referenced in the following as terrain conductivity meters (TCM""s) and multi-frequency EM sounders (MEMS). Extensions to the TCM approach, which will be referenced as the ground conductivity meter (GCM) and array conductivity meter (ACM), improve aspects of TCM operation.
TCM""s
The first devices which could be described as TCM""s were described by Doll (Doll, H. G. 1949, Introduction to Induction Logging and Application to Logging of Wells Drilled with Oil Base Mud, J. Pet. Technol. 1, pp 148-162) in a borehole context and Howell (Howell, M., 1966, A Soil Conductivity Meter, Archaeometry 9, pp 20-23) in a shallow soil conductivity measurement context. Examples of commercially-available TCM""s include the Geonics Ltd. EM-31, EM-34 and EM-38 and the Geoftzyka CM-031. These devices use the Low Induction Number Approximation (LINA) to estimate the apparent conductivity of the earth materials over a range of depths by a linear scaling operation from the component of the reflected EM signals which are in quadrature (i.e. at a 90xc2x0 phase shift) with the primary field emitted by the sensors transmitter coil. The component of the EM measurement arising from signals in phase with the primary field from these devices may also be interpreted, with some effort, in terms of apparent conductivity, as well as the apparent magnetic permeability of the earth materials under test. The terms apparent conductivity and apparent magnetic permeability are defined below.
Existing TCM""s incorporate a transmitter coil transmitting a sinusoidal signal at a single stable frequency (e.g. approximately 10 kHz for the EM-31) such that the LINA holds, i.e. that the following inequality is true:
[(xcfx89xcexc"sgr"0)xc2xdxcfx81] less than 0.5
where xcfx89 is the operating frequency in radians/sec, xcexc is the magnetic permeability of the earth in henrys/metre, "sgr"0 is the conductivity of the earth in Siemensim, and xcfx81 is the separation in metres between the transmitter and receiver coils (as described further below).
In a TCM, a receiver coil is located at a distance xcfx81 from the transmitter coil and substantially coplanar with the transmitter coil. There may or may not be a preamplifier located near this receiver coil to increase its effective output signal level. The peak moment (transmitter coil current times number of turns time area of one turn) of the transmitter coil""s magnetic field is such that an acceptable signal to noise ratio (SNR) can be obtained at the receiver.
TCM""s also incorporate electronics which can analyse the signal picked up at the receiver coil into components in phase and in quadrature (90 degrees out of phase) with the transmitted field. The quality of calibration of the output of this process and its stability are important factors in the utility of the instrument. These electronics also include circuits and/or software which convert the measured quadrature component into an apparent resistivity using the LINA relationship as stated in McNeill (McNeill, J. D., 1980, Electromagnetic Terrain Conductivity Measurement at Low Induction Numbers, Technical Note TN6, Geonics Limited, Mississauga, Canada) after Wait (Wait, J. R., 1962, A Note on the Electromagnetic Response of a Stratified Earth, Geophysics 27, pp 382-385.), i.e.
"sgr"a=4/(xcfx89xcexcxcfx812)*(Hs/Hp)quadrature
The fundamental unit of apparent conductivity under the MKS system of units is Siemensimeter (S(m), although most if not all TCM""s present their quadrature data in terms of milliSiemensim. In situations displaying horizontally-layered geology, the apparent conductivity represents a weighted average of the earth materials"" conductivity in the vicinity of the sensor. A commonly-accepted rule-of-thumb depth of investigation (DOI) has been defined by McNeill as 1.5 times the transmitter-receiver separation for the horizontal coplanar configuration and 0.75 times this separation for the vertical coplanar configuration, corresponding to a cumulative response value of approximately 30%. Using the same 30% value for the perpendicular configuration""s cumulative response yields a DOI for this configuration of approximately 0.5 times the transmitter-receiver separation.
A separate data logging device is typically provided which can acquire, store and display the analog outputs of typical commercial TCM""s on demand or at a preset sampling rate.
The Transmitter and Receiver coils are typically installed near the ends of a tubular boom or other support structure, while the electronics and data logging device are mounted in a package near the central point of the support structure. The boom in the EM-31 is designed to be partially disassembled for shipping, with the two outer portions attached via couplings to the central portion of the boom, which is affixed to the electronics package. Short-offset systems like the EM-38 are housed in unitary support structures which incorporate the receiver electronics. The EM-34 does not incorporate a rigid housing joining the transmitter and receiver.
The orientation of the Transmitter and Receiver coils is such that the axis of each coil is approximately vertical when the instrument is held in an upright position (the Horizontal Coplanar orientation).
TCM""s incorporating horizontal coplanar coil geometries can be rotated 90 degrees about a line joining the Transmitter and Receiver coils to place the coils into the Vertical Coplanar orientation, wherein the axes of the coils are horizontal. As described above, this approximately halves the effective DOI for the system, and allows the user to investigate vertical variations In the conductivity structure of the earth.
The principal shortcomings of the TCM are its single transmitter-receiver coil pair, which doubles the measurement time per station if two depths of investigation are desired at each site, its substantial weight (12.4 kg for the EM-31), its weak joint structure (for the EM-31), which permits substantial sag and flexibility in the boom when assembled, and its bulky packaging-which generates uncomfortable magnitudes and directions of pressure on the operator""s shoulder. TCM""s which rely exclusively on the LINA formula quoted above for estimation of earth material conductivity will generate erroneous values when used under very conductive conditions.
MEMS
A related class of non-contacting multi-frequency electromagnetic sounders (MEMS) used for near-surface earth material investigation measure the variation in the instruments electromagnetic coupling with earth materials as a function of frequency. Examples of such instruments include airborne electromagnetic (AEM) sensors developed by various companies over the years, including Barringer Research, Dighem, Geotech, Geoterrex, Geophex, and Aerodat, and ground systems such as the Apex Double-Dipole(trademark) and the Geophex GEM-2(trademark) and Geophysical Survey Systems"" GEM-300(trademark) which operate in the range 330 to 20,000 Hz.
Conventional AEM sensors have been thoroughly described in the literature (e.g, Palacky and West, 1987). Over the last fifteen or twenty years, efforts have been made to increase the quantitative capabilities of some AEM sensors through improvements to calibration methodologies and the introduction of electronic calibration methods. These efforts achieved encouraging, though not definitive, results. The multi-frequency, rigid-boom approach used in helicopter electromagnetics (HEM) received the most attention in terms of calibration. In their simplest form, HEM sensors incorporate a linear coil array consisting of a transmitter, a receiver of effective area (turns times single-turn area) A, and a bucking coil possessing an effective area Axcfx813, where xcfx81 is the relative position of the center of the bucking coil between the center of the transmitter (at xcfx81=0) and the center of the receiver coil (at xcfx81=1). The bucking coil is connected in opposition to the receiver coil, so that the signal induced in the combined receiver-bucking coil circuit by the primary field generated by the transmitter is reduced to approximately zero. The secondary EM signal, due to eddy currents induced in nearby electrically conductive media by the primary field, is the quantity to be measured by the sensor array. This bucking method reduces the dynamic range requirements placed on the amplification and signal-processing electronics used to acquire and analyse the secondary EM signal.
The method, as described to this point, is the same for narrowband or wideband HEM sensors. Narrowband sensors transmit sinusoidal signals at one frequency per set of transmitter, receiver and bucking coils, so that multiple-frequency measurements require multiple sets of these coils. Wideband sensors broadcast a more complicated waveform, which includes many frequency components. The amplitude and phase of the secondary signal (defined with respect to the amplitude of the primary signal at the receiver location) are, in most narrowband implementations, measured through analogue or digital synchronous demodulation of the signal using analog signal processors, where the phase reference for the demodulation process is obtained from a reference coil positioned near the transmitter coil. The amplitude information present in the phase reference signal is discarded by conventional HEM signal processor units.
In wideband sensors, the bucked signal is typically digitized and either stacked (i.e. each period of the waveform is added together to yield a stacked waveform as described by Becker and Cheng (1987), or stored in memory for later analysis. A reference waveform representative of the primary field is also normally stacked or stored. The stacked or stored waveforms are then subjected to Fourier deconvolution, in which the Fourier transform components of the signal waveform are divided by the corresponding components of the reference waveform. For example, in the GEM-300(trademark) ground EM sensor, the bucked signal and the reference (obtained from the bucking coil) are digitised into parallel data streams. The EM response at each operating frequency is computed through convolution with cosine and sine data series at that frequency (effectively the computation of Discrete Fourier Transformation coefficients for these frequencies), followed by correction for amplifier gains, coil geometry and the amplitude and phase of the transmitted signal measured using the reference signal (Won, I. J., D. A. Keiswetter, G. R. A. Fields, and L. C. Sufton, 1996, GEM-2, A New Multifrequency Electromagnetic Sensor, JEEG 1, pp. 129-137). The resulting EM response data are either recorded directly or converted to an apparent conductivity value through a transformation relating EM response to the product of apparent conductivity and frequency (Won et al, 1996). The GEM-300(trademark) instrument reportedly suffers from deficiencies in calibration and zero-level stability, which make it difficult to use their measurements for quantitative layered-earth interpretation (Nyquist, Sageep reference). It also lacks the frequency range necessary for unambiguous resolution of earth material property layering within the first two metres below surface except in extraordinarily conductive conditions (McNeill, Geonics TN30). For example, at an operating frequency of 1 MHz (about 50 times higher than its actual maximum frequency), such an Instrument could not resolve layered structures smaller than about 1 metre, even in extremely conductive 0.2 S/m earth materials.
GCM
The ground conductivity meter as defined here is an extension to the asic horizontal-coplanar geometry TCM through the addition of a second receiver coil mounted at right angles to the first one so that this axis of this second coil precisely intersects the transmitter coil. An instrument incorporating such a receiver coil in addition to a horizontal-coplanar (HCOP) receiver coil is the subject of a Canadian Patent 2,142,546 issued Apr. 13, 1999 to Richard S. Taylor and entitled Apparatus and Method for Sounding the Earth. This alternative transmitter-receiver orientation is known as the Perpendicular Loop configuration (PLC). The output of this PLC coil is sensitive to ground conductivity variations to a depth approximately one-third of that to which the horizontal coplanar coils are sensitive for a horizontally-stratified earth material properties distribution. By using both the HCOP and PLC configuration outputs, one can simultaneously estimate ground conductivity for two DOI""s. The first DOI extends to 1.5 times the transmitter-receiver coil spacing for the horizontal coplanar coil pair, while the second DOI extends to 0.5 times the coil separation for the perpendicular coil pair.
These simultaneous measurements permit continuous EM profiling at two depths of exploration, an important improvement in terms of productivity over the TCM. Another advantage of the PLC extension is that adding a second receiver coil and its attendant electronics to a single-transmitter instrument is more efficient in terms of power usage and weight, providing a competitive advantage over devices which seek to monitor different depth ranges using multiple transmitters and receivers via MEMS technology.
The term Earth materials should be interpreted as inclusive of materials including but not limited: to soils; rocks; minerals; ores; ice; and solvents such as water, brine, pore fluids, ammonia and methane; located on or in the earth or extraterrestrial bodies including planets, moons, asteroids or comets. Earth materials may be characterised by their electromagnetic (EM) properties, i.e. their electrical conductivity and permittivity, and their magnetic permeability. The distribution of these EM properties with respect to depth and position may be interpreted in terms of more geologically or geotechnically useful quantities, including composition, porosity, degree of fluid saturation, concentration of salts or other conductive species in the solvent, and the concentration of magnetically permeable materials.
This invention provides an improved means for quantitative estimation of the electromagnetic properties of earth materials with a lightweight, self-contained apparatus. In some embodiments the apparatus is capable of stable extended operation at low power levels, while in others low-power operation may be sacrificed for the sake of reduced noise levels or extended spectral range.
The requirement for quantitative measurement of earth material properties arises in many applications, most of which lie in the fields of geotechnical engineering and environmental assessment, mining, or industrial processes. The invention will improve the quality and acquisition rate of measurements of the electromagnetic properties of near-surface earth materials, buried objects, bodies, voids or other agglomerations of material of natural or technological origin possessing a contrast between their electromagnetic properties and the surrounding material.
Some particular applications, which may include either surface-based or low-altitude airborne measurements, include:
rapid profiling and display of depth-conductivity profiles;
measurement of the electrical conductivity and thickness of floating ice (or other solids) and/or of the underlying water (or other conducting fluid);
monitoring of soil and/or groundwater salinity for agricultural purposes;
monitoring of near-surface geology, moisture content, salinity and electrolyte
pollution of swamps, marshlands and wetlands;
searching for objects such as vehicles or structures buried by natural disasters such as avalanches and landslides;
detection of unexploded ordnance;
detection of underground or underwater storage tanks, pipes, transformers and other highly conductive structures; and
detection of archaeological features such as walls, pits, hearths, floors, postholes, middens, mounds and artifacts;
Accordingly to one aspect of the invention, an electromagnetic sensing apparatus comprising an electromagnetic transmitter, at least one receiver device, at least one calibration device, a switch and a processing means. The electromagnetic transmitter generates a transmitter signal. The receiver device is spaced from the transmitter, senses a receiver local electromagnetic field proximate thereto and generates a receiver signal. The calibration device senses a calibration local electromagnetic field proximate thereto, generates a calibration signal. The calibration device is positioned such that the distance between the transmitter and the receiver device is greater than the distance between the transmitter and the calibration device. The switch is connected between the receiver device and the calibration device. The processing means includes a first input for receiving a signal from one of the receiver signal and the calibration signal and a second input for receiving a signal from the switch. The processing means compares the first input and the second input and monitors distortions in the transmitter signal. The sensing apparatus may include a plurality of receiver devices in a variety of different configurations.
Accordingly to another aspect of the invention, an electromagnetic sensing apparatus comprises an electromagnetic transmitter, a first receiver device, a second receiver device and a processing means. The electromagnetic transmitter generates a transmitter signal. The first receiver device is spaced from the transmitter, senses a first receiver local electromagnetic field proximate thereto and generates a second receiver signal. The second receiver device is spaced from the first receiver device, senses a second receiver local electromagnetic field proximate thereto and generates a second receiver signal. The processing means has a first input for receiving the first receiver signal and a second input for receiving the second receiver signal. The processing means compares the first input and the second input and monitors distortions in the transmitter signal.
Accordingly to a further aspect of the invention, a method of analysing data from electromagnetic sensing apparatus comprises a plurality of steps. A receiver signal from a receiver device is received and converted into a complex receiver signal. Similarly a calibration signal from a calibration device is received and the converting signal to a complex calibration signal. The ratio of the complex receiver signal to the complex calibration signal is adjusted for the signal processing components to produce a processing adjusted complex ratio. The processing adjusted complex ratio is adjusted for the effective areas of the receiver device and the calibration device to produced an area adjusted complex ratio. The area adjusted complex ratio is adjusted for a distance between the receiver device and a transmitter and the distance between the calibration device and the transmitter to produce a calibrated field coupling ratio.
Further features of the invention will be described or will become apparent in the course of the following detailed description.